Aging mice can be made "young" again, according to findings one scientist initially found unbelievable.

The key is muscle-derived stem cells, which—like other stem cells—are unspecialized cells that can become any type of cell in the body.

When injected with muscle stem cells from young mice, older mice with a condition that causes them to age rapidly saw a threefold increase in their life spans, said study co-author Johnny Huard, a stem-cell expert at the McGowan Institute for Regenerative Medicine in Pittsburgh.

"I've been doing science for the last 20 years," Huard said. What "makes the story so amazing is that in the beginning, I didn't believe the result," he said.

"I bet that we mixed up the animals—you know, scientists are always skeptical."

"Tired" Stem Cells Reenergized

The study mice were genetically engineered to have a condition similar to a rare human syndrome called progeria, in which children age quickly and die young. (Learn more about the human body.)

The fast-aging mice typically die around 21 days after birth, far short of a normal mouse's two-year life span.

When scientists looked at the muscle stem cells of the fast-aging mice, they found what Huard called "tired" stem cells, which don't divide as quickly.

The team then examined mice that had aged normally and found their stem cells were similarly defective.

Curious if these deficient stem cells contribute to aging, Huard and colleagues injected stem cells from young, healthy mice into the fast-aging mice about four days before the older animals were expected to die.

To Huard's astonishment, the treated mice lived an average of 71 days—50 more than expected, and the equivalent of an 80-year-old human living to be 200, he said.

Not only did the animals live longer, they also seemed healthier, the scientists found.

The "drastic" results bore out with repeated experiments, leaving the scientists to wonder how exactly the stem cells were working their magic, Huard said.

To find out, the team "tagged" stem cells injected into the fast-aging mice with a genetic marker that tracked where the cells went inside the body. Surprisingly, the team found only a few stem cells in the mouse organs, squashing a theory that the introduced cells were repairing organ tissues.

The scientists went back to the lab to test another idea: that stem cells secrete some kind of mysterious anti-aging substance.

The team put stem cells from the fast-aging mice on one side of a flask and stem cells from normal, young mice on the other side. The two sides were separated by a membrane that prevented the cells from touching.

Within days, the aging stem cells began acting "younger"—in other words, they began dividing more quickly.

"We can conclude that probably normal stem cells secrete something we don't know that seems to improve the defects in those aging stem cells," Huard said.

"If we can identify that, we have found an anti-aging protein that is going to be important" for people, said Huard, whose study appeared January 3 in the journal Nature Communications.

Stem-Cell Research "Intriguing" but Preliminary

But other scientists are cautious about how soon the discovery may help people delay the aging process or treat age-related disease.

"They did a beautiful job of showing that, when they put the muscle stem cells in [the mice], they improved function," said Justin Lathia, an assistant professor of cell biology at the Cleveland Clinic's Lerner Research Institute.

But as far as people go, it's still not clear what exactly stem cells do in the body, as well as what the mysterious stem cell secretion really is, Lathia emphasized.

Jeremy Rich, chair of the department of Stem Cell Biology and Regenerative Medicine at the Cleveland Clinic, also pointed out that the study is limited to muscle stem cells. That means the research can't be generalized to include all stem cell types, which are often very different from each other.

Paul Frenette, a stem cell and aging expert at the Albert Einstein College of Medicine in New York, called the research "intriguing," but said one of the messages for "patients is not to get too excited."

"You see all these clinics that are popping up all over the world—even in New York—where they're injecting stem cells" into people to treat disease, even though such therapies have not been proven.

"I don't think people should run to the clinic right now to have injections of stem cells to live longer."